By the end of 2011, two thirds of U.S. airline passengers will be asked to step through one of 1,000 new Rapiscan X-ray machines. But some scientists are concerned about the unprecedented radiation exposure coming along with them.

Specifically, they're worried about the half of the machines that are classified as back-scatter scanners, which use low-energy X-rays to peek underneath passengers' clothes. Such machines are already in place at 23 American airports, but by the end of next year they'll be a security standard.

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That has many scientists very concerned. A group headed by a biochemist from the University of California, San Francisco, says that any ionizing radiation can potentially lead to chromosome damage and, eventually, cancer. He and his colleagues have written a letter to the President's science advisor urging for further studies on the effects of radiation from the new scanners.

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David Brenner, who heads Columbia University's Center for Radiological Research, doesn't think that the small dose of radiation from the scanners poses much of a risk to most individuals, but the scale of the operation—and the fact it will include a small percentage of the population who may be unwittingly sensitive to radiation—is problematic. NPR explains:

"There really is no other technology around where we're planning to X-ray such an enormous number of individuals," Brenner told the caucus and congressional staffers. "It's really unprecedented in the radiation world..."

Recent research, Brenner says, indicates that about 5 percent of the population - one person in 20 - is especially sensitive to radiation. These people have gene mutations that make them less able to repair X-ray damage to their DNA. Two examples are the BRCA-1 and BRCA-2 mutations associated with breast and ovarian cancer, but scientists believe many more such defects are unknown.

"I don't know if I'm one of those 5 percent. I don't know if you're one of those 5 percent," Brenner says, "And we don't really have a quick and easy test to find those individuals."

TSA and FDA officials say that the radiation from the new Rapiscan machines is negligible, and point out that scanning is technically optional, but it's hard to take their word—which can't totally be divorced from the multi-million dollar government investment the machines entail—over that of the leading researchers in the field. Just file it in the back of your mind as one more thing to worry about when you're flying next year. [NPR via Dvice]